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Certain shark species are more likely to die as bycatch

The varied breathing modes of different shark species affect the chances of them dying during fisheries capture, a new study suggests.

A team of researchers from Monash University, analysed data on over 80 species of shark and ray that were accidentally caught by commercial fishing techniques including longlines, gillnets, and trawls. When this occurs, the caught individuals are referred to as bycatch and it arises due to many fishing methods and gear not being selective and subsequently results in the catching and discarding of millions of unintentionally caught fish. The Elasmobranchs, a subclass of Chondrichthyes or cartilaginous fish that include the sharks and the rays and skates, are some of the most frequent bycatch found in commercial fishing gear.

Lead researcher, Monash University PhD student Derek Dapp explained that approximately 100 million sharks are caught worldwide each year, with many of these accidentally captured by fisheries targeting other species of fish.

“Sharks can die during capture owing to physical injury or suffocation, but until now the sensitivity of different species has not been well understood,” Mr Dapp said.

“A quarter of shark and ray species are threatened with extinction, so urgent action is needed to improve their chances of survival.“

The team’s research showed that species such as gummy sharks that are able to breathe while resting using a special respiratory opening called a spiracle, generally survived much better than those that need to continuously swim forward to breathe such as hammerhead and mako sharks.

Bycatch: A thresher shark, silky shark and bat ray

Joint lead researcher Associate Professor Richard Reina from Monash explained why active species, such as mako sharks, are more likely to die during capture.

“All commonly used commercial fishing techniques reduce a shark’s ability to swim forward, so as soon as they are captured, active sharks begin to suffocate on fishing gear. Active sharks are unlikely to survive when they experience long capture times because they are unable to breathe effectively, while less active species that use a spiracle are much more tolerant to prolonged capture,” Associate Professor Reina said.

The differing methods of how sharks obtain oxygen results from some (older) species breathing using buccal pumping. This method gets its name from the buccal (mouth) muscles that actively draw water into the mouth and over the gills, allowing the sharks to respire while remaining still. These sharks also have prominent spiracles, or respiratory openings behind the eyes that allow the fish to pull in water while buried under sand. Species including nurse sharks, angel sharks, skates and rays breathe this way. These species tend to spend most of their time lying on the bottom of the ocean floor.

Other shark species use ram ventilation, where they ventilate their gills by swimming with their mouths open. Some sharks, such as the tiger shark, can switch between buccal pumping and ram ventilation depending on quickly they’re swimming. As sharks evolved and became more active, this method of pumping became secondary.

“Obligate ram ventilators” however are sharks that have completely lost the ability, and the necessary anatomy, for buccal pumping and instead can only respire using ram ventilation. Sharks from this group (which includes great white, mako and whale sharks) would indeed die from lack of oxygen if they stopped swimming.

Some sharks must swim continuously so oxygen can be extracted from the water as it is driven through their mouths and over their gills. This process is known as ‘ram-ventilation’.

The varying methods utilised in commercial fishing have shown to affect the number of sharks accidentally captured. Tow netting and long line fishing have been shown in some cases to result in shark bycatch of 74.9% of the total catch for tow netting and a 70% bycatch in long line fishing. The cumulative effects of stress and physical injury to elasmobranchs caught in fishing mechanisms has been shown to result in high mortality rate of the bycatch. The most common elasmobranchs found in bycatch on pelagic long lines exhibit significantly high mortality rates, which include the dusky (Carcharhinus obscurus) with 48.7%, silky (Carcharhinus falciformis) with 66.3% and night sharks (Carcharhinus signatus) with 80.8%.

“There are over 1000 species of sharks and rays, but the chance of death during fisheries capture is estimated for less than 10 per cent of these species.” says Reina.”Our research identifies a very large group of sharks potentially vulnerable to fisheries capture.

“Now that we’ve identified why some species are more likely to die during capture, we can begin to work with fisheries managers to design fishing strategies that reduce the impact on these species and halt the overall decline in shark populations.“

The team’s findings were published in the leading fisheries journal, Fish and Fisheries.